This invention relates to a filtration method and a filter press for employing the method, and more particularly, to a filter press in which the lower opening of a tubular filter medium is closed so that the medium may be formed into a sack before liquid to be filtered is fed into the medium through the upper opening thereof, and the capacity of a filter chamber formed in the medium is reduced, with the result that the expressing filtration of the liquid is performed to produce a cake with a desired water content in the chamber.
Conventionally, there has been proposed a filter press in which a pair of filter cloths are arranged between adjacent filter plates and a filter chamber is formed between the pair of the cloths arranged between filter beds of the adjacent plates in closing the plates so that liquid to be filtered may be fed into the chamber for filtration.
However, in the arrangement thereof, the filtration area is kept constant due to the size of a gap between the beds of the adjacent plates and the configuration of the chamber is limited by that of the gap, resulting in a small degree of freedom of the cloth to fluctuate in configuration with respect to the cake.
Then, for example, there has been proposed a filter press in which after the lower opening of a tubular filter medium is closed, liquid to be filtered is fed into the medium through the upper opening thereof to filter the liquid, and after completion of the filtration, the lower opening of the medium is opened to discharge the cake from the medium.
However, in the construction thereof, since the lower opening of the medium is only opened in discharging cake, cake attached to the medium can not be discharged from the medium. In order to remedy the problem, it is possible to consider that the medium is subjected to vibration by a manual operation. However, the operation is troublesome and it is difficult to give the vibration to the whole medium in a large filter press.
On the other hand, there has been proposed a filter press, as an expressing type filter press, in which expressing type filter plates having pressing diaphragms and filter plates without pressing diaphragms are alternately arranged between a front and a rear stand serving as fixed plates to be capable of moving backward and forward therebetween. A pair of filter cloths are arranged between the expressing type filter plate and the filter plate which does not have the diaphragm adjacent thereto, the plates being moved toward the front stand by a movable plate under pressure to form a filter chamber defined between the pair of the cloths arranged between the adjacent plates so as to feed liquid to be filtered into the chamber for filtration.
In this filter press, in the case where the movable plate is driven for expressing filtration, the movable plate continues to be driven for the expressing filtration until the pressure caused by the movable plate reaches a specified expressing pressure. Therefore, it may be impossible to sufficiently perform the filtration of liquids having certain characteristics. For example, in the case where a liquid to be filtered (including fruit, etc.) is expressed, since cake in the liquid, that is, the peel of the fruit, adheres to the inner surface of the cloths to form a film, cake with a desired water content can not be obtained even under high expressing pressures. Moreover, in a filter press employing a pressing diaphragm, since the diaphragm is normally made of rubber, the rubber diaphragm can not be used for filtration of corrosive liquids to be filtered which corrode rubber and should be replaced with a corrosion-resistant diaphragm. However, the corrosion-resistant diaphragm has small strength, with the result that it is very difficult to obtain cake with a desired water content from the corrosive liquid. Furthermore, in the filter press employing the diaphragm, the control of the water content of the cake is not performed on the basis of the directly measured expressing pressure in the chamber but is performed on the basis of the pressure of the compressed air for expanding the diaphragm, with the result that the control is indirectly performed. Therefore, the difference between the pressure of the air and the actual expressing pressure becomes larger.